Session 8.6 Diurnal cycles of thermally driven flows in two adjacent Utah valley systems as revealed by EOF flow patterns

Tuesday, 22 June 2004: 2:45 PM
Francis L. Ludwig, Stanford University, Stanford, CA; and J. Horel and C. D. Whiteman

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Empirical orthogonal functions (EOFs), determined from three sets of stations in valleys near Utah’s Great Salt Lake, were used to define major flow patterns and their diurnal cycles. Data were selected to represent conditions conducive to thermally driven flows, i.e weak synoptic forcing and clear skies. The data sets were comprised of 8 or 10 stations, mostly routinely operated, but some data came from the Vertical Transport and Mixing experiment (VTMX) of October 2000. The primary EOFs represented flows that were directed predominantly along valley axes and were caused by channeled or thermally driven flows. Diurnal variations in EOF intensity showed that thermal effects were the most common causal mechanism. The along-valley EOFs accounted for 43 to 58 percent of the variance in the three wind component data sets that were used. The second EOFs accounted for 13 to 18 percent of the variance.

The significance of correlations between intensities of the major flow patterns in the adjacent valleys will be discussed. We also will examine the diurnal cycles of these flow patterns and how they vary from day to day, An example of the “typical” cycle will be shown. One case study shows how a frontal passage manifests itself in the flow pattern intensities. The EOF approach shows considerable promise for classifying wind patterns and selecting or otherwise defining representative cases for simulation or for further detailed analysis.

This work was supported in part by the U.S. Department of Energy, under the auspices of the Atmospheric Sciences Program of the Office of Biological and Environmental Research.

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